DESCRIPTION: (Applicant's Description) Exposure to organochlorine pesticides and polychlorinated biphenyls (PCBs) is pervasive due to widespread use in the past and environmental persistence of these ubiquitous lipophilic compounds. Recent epidemiologic studies suggest that exposure to some of these chlorinated hydrocarbon pesticides and PCBs, which mimic the biologic effects of estrogens, increase the risk of breast cancer. The primary aim of this study is to determine the feasibility of establishing a substantial cohort of women with historical data on body-burden levels of chlorinated hydrocarbon pesticides and PCBs from the: (a) 1,436 participants of the First National Study to Determine Levels of Chlorinated Hydrocarbon Insecticides and PCBs in Human Milk, conducted between 1974 and 1976; and (b) 1,842 participants of the second National Study to Determine Levels of Chlorinated Hydrocarbon Insecticides and PCBs in Human Milk, conducted between 1977 and 1983. Such a cohort would provide a unique opportunity to more fully evaluate the effects of past exposure to chlorinated hydrocarbon pesticides--both individually and as a chemical mixture--on the incidence of breast cancer; as well as to study the health of offspring exposed to these compounds through lactation. In the first phase of the feasibility study, 50% (1,640) of the 3,278 former study participants will be randomly sampled, after stratification for demographic variables, and traced for address location. Located subjects of this cohort will be contacted by letter with telephone interview follow-up to determine participation and to complete a questionnaire on current health status, specifically regarding incidence of breast cancer. Data on the incidence of breast cancer among the proposed cohort will be evaluated with respect to historical levels of chlorinated hydrocarbon compounds in breast milk via multiple Poisson regression analysis with adjustment for potential confounders. Additionally, because the statistical analyses of the data from each of the two national studies had been limited in scope due to the nature of these biomonitoring studies, a specific aim of this research is to thoroughly analyze the combined data from the two national studies with particular attention to both the relationships among the residues of organochlorine insecticides and PCBs and the relationships between these residues and explanatory epidemiologic factors.